A turbofan engine comprises an engine core about which is disposed a fan casing. Generally, the fan casing is coaxial with the engine core and is supported on the core by a main support structure extending between the fan casing and the core. In addition to the main structure, the rear of the fan casing may be secured to the engine by struts which extend between the engine core and the rear of the fan casing. The struts resist relative motion between the engine core and the rear of the fan casing. The arrangement of the struts, fan casing and fasteners securing the struts to the fan casing must be able to withstand extreme loads such as those generated during a fan-blade-off event (i.e. shedding of a fan blade), for example following a bird strike.
The fan casing may also be attached to other functional elements such as thrust reverser units.
In order to improve rigidity of the fan casing it is known to provide the fan casing with a stiffener flange which extends radially outwardly from the fan casing. The stiffener flange is aligned with the struts in a common radial plane so that the forces acting at the root of the flange are transmitted directly through the struts. Typically, the fan casing is provided with hollow dowels which extend through bores in the casing. The dowels resist transverse loading of the fan casing (i.e. rotational and axial loading of the casing). In addition, fasteners extend through the dowels and secure the casing to the struts in the radial direction. The fasteners resist radial loading of the fan casing. The fasteners and dowels are arranged on opposing sides of the stiffening flange. This ensures an even distribution of load about the stiffener flange. The bores in the casing have to be machined with precise diametral tolerance. With composites there has been found to be fibre relaxation which makes it very difficult to achieve the required diametral tolerance to receive the dowels. Composite fibres also reduce in strength significantly when exposed to high temperatures and are liable to deform and tear in the event of an engine fire local to the strut interface under engine loading.